A system has been developed to study physical structure and regulation of expression of an endogenous murine xenotropic type C virus of NIH Swiss mice. The system is a transfection assay using the CaCl2 procedure of Graham and van der Eb along with DMSO in mink and cat recipient cells. Only complete proviral units from sheared cellular DNA are detected, and the virus produced, at least in the mink cells, is that coded for by the provirus. Cat-mouse recombinants may occur in cat cells. Agarose gel electrophoresis has been used to separate and size DNA fragments. 12-O-tetradecanoyl-phorbol-13-acetate (TPA) is being studied, as a known modifier of cell membrane structure and as a virus inducer, for possible modifying effects on the transfection process. TPA also markedly enhances the incorporation of labeled thymidine into acid precipitable product in mouse lymphocytes. The mechanism of this is being studied with special attention to the activities of several DNA polymerases and thymidine kinase.